Desmear chemicals, such as sodium or potassium permanganate, chromic acid or sulfuric acid are well known in the printed circuit board industry for cleaning of the drilled through-going conduits in the printed circuit board production. Especially, if sodium or potassium permanganate have been used for such a desmear cleaning procedure, there are generated large amounts of desmear particles by the chemical reaction of the desmear chemical with resin residues at the sites of the drilled through-going conduits of the printed circuit board.
Said generated desmear particles are in case of sodium or potassium permanganate manganese dioxide particles, which are known to agglomerate on the bottom of the respective desmear container of a desmear module.
However, it is often difficult to mechanically remove said desmear particles again from the bottom of the respective desmear container, in particular only by desmear liquid flow; and especially to remove them in a continuous manner during the horizontal processing of the respective substrates to be treated due to the high density of the suspension of manganese dioxide particles in the desmear treatment liquid.
There have been also attempts in the past to remove said manganese dioxide particles by a chemical reduction process by making use of suitable additional chemicals. But, the purpose of said invention has been to find a new and inventive mechanical solution for removing said desmear particles.
Attempts up to now to mechanically remove said desmear particles have only been partially successful and have been plagued with problems arising from the disadvantageous need to stop the entire horizontal processing line to allow a separate cleaning of the desmear container. Such a break leads to an increased interrupting time and to a loss of efficiency, which again leads to higher cost of such horizontal processing lines.
In order to alleviate this problem, there have been installed complex automatic cleaning procedures in the past in order to avoid interrupting the horizontal processing of the substrates to be treated due to maintenance issues. But, such solutions thus typically increase the cost and complexity of such horizontal processing lines.
In document DE3813518 A1 disclosed is a device for mechanically removing such desmear particles. Herein, a pair of nozzles, namely a pressure nozzle arranged directly above a drilled through-going conduit and a suction nozzle arranged directly below the same drilled through-going conduit, has been used for directly pressing desmear treatment liquid through the drilled through-going conduit, wherein the passing desmear treatment liquid comprising desmear particles of the respective drilled through-going conduit enters directly the suction nozzle. Thus, the desmear particles can be removed directly from the desmear module. But, such a solution has been only found suitable for thick printed circuit boards, which provide a sufficient inherent rigidity. The substrates to be treated in our days are much more challenging due to their reduced thickness and increased flexibility, which prevents the application of such a procedure.
Another problem encountered in common horizontal processing lines is the need of providing a large desmear container with a high desmear container volume, to avoid that desmear particles enter an intake area of a lower part of a required immersion pump. Thus, a large desmear treatment liquid volume has to be provided, which makes the process costly again.